The Obama administration will seek to cut greenhouse-gas emissions from existing power plants 30% by 2030, potentially one of the biggest steps any country has ever taken to confront climate change, people familiar with the plan said Sunday.

Seen as the linchpin to President Obama’s climate-change campaign and a key part of his legacy, the proposed power-plant rule will set state-specific targets for carbon-dioxide reductions and let local officials decide how best to meet the goals.

A formal announcement is expected Monday morning, but details began leaking out Sunday. Because the rule has not been officially issued yet, the 30% target reduction could be one of several options the Environmental Protection Agency presents for public comment, environmentalists said.

The proposal will make good on a pledge the president made last year to tackle climate change through his executive authority in the face of congressional inaction on the issue.

The rule targets the nation’s fossil-fuel-burning power plants, which emit 40% of heat-trapping carbon dioxide, more than any other source. States will be asked to reduce carbon-dioxide emissions 30% from their levels in 2005, when emissions were 10% higher than they were in 2012.

States that burn a lot of coal would begin their reductions from a higher emissions level than those that burn natural gas, which emits less carbon dioxide.

The proposed rule would allow states to offset their power plants’ greenhouse gas emissions by developing other parts of the electricity system, such as encouraging energy efficiency or greater use of renewable energy.

Foes of the rule -- such as the U.S. Chamber of Commerce, congressional Republicans and Democrats from coal states -- have already lined up to oppose it as government overreach that would do little to address climate change while imperiling the economy.

“Now the president is once again looking to do through regulation what he couldn't accomplish through legislation. But myself and others are sounding the real alarm of how the president's plan will be dangerous for our economy and future job opportunities,” said Sen. James Inhofe (R-Okla.), citing a chamber report that estimated the rule could cost the economy $50 billion annually.

The administration’s allies countered that the rule offers states that have a lot of coal generation the flexibility to keep running the most efficient plants, although at lower capacity, which would ensure reliability and affordable rates. The reduced coal-fired output could be made up through energy efficiency and renewable power.

“Our industries see the release of EPA's proposed carbon standards for existing power plants as a historic event,” said Lisa Jacobson, president of the Business Council for Sustainable Energy. “Making these standards work will require giving states flexibility to use the wide range of available technologies, from energy-saving upgrades to affordable natural gas and renewables. These technologies have never been cheaper or more widespread.”

CHICAGO — On Monday, President Obama plans to unveil what is likely to be the most significant initiative of his presidency to combat climate change: a new rule to cut carbon dioxide emissions from existing power plants, the country's single greatest source of the heat-trapping gas.